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The Glycemic Index (GI) is a system for rating carbohydrates, or saccharides, based on their immediate effect on the blood glucose level. An organic compound, glucose is a simple sugar or monosaccharide. Glucose is the principle sugar circulating in the blood; all cells use glucose as a source of energy and as a metabolic intermediate. Found in plants, glucose is the product of photosynthesis (the process of converting light energy to chemical energy and storing it in the bonds of a glucose molecule). Biologically active glucose is often referred to as dextrose. Glucose is often incorrectly referred to as table sugar – table sugar is a disaccharide composed of glucose and fructose.
When food containing carbohydrate is consumed, glucose from the food is digested and transported throughout the body in the blood. The speed with which the food is able to increase the blood glucose level is referred to as the glycemic response. The glycemic index of a food is defined as the blood glucose level measured over a 2 hour period after ingestion of a fixed portion of carbohydrate and expressed as a percentage of the area under the glucose response curve when the same amount of carbohydrate is consumed as glucose [(1].
Thus, carbohydrates that break down rapidly during digestion raise the blood glucose level quickly have a higher GI rating than foods that break down more slowly and raise the blood glucose level less. The glycemic index is influenced by many factors, including how much food is consumed, how the food is prepared and what other types of food were consumed along with the carbohydrate. For example, the ingestion of fiber with carbohydrate tends to flatten the glycemic response, likely due to the fiber reducing the absorption rate of the carbohydrate [(2]. The glycemic load is another rating system that takes into account both the glycemic index and the amount of carbohydrate consumed.
Why all the fuss? Low GI foods are usually lower in calories and fat, and high in fiber and nutrients. Low GI diets have been associated with increased HDL cholesterol levels (the good cholesterol) and a decreased risk of developing diabetes and cardiovascular disease [(3]. The blood glucose level can also affect how hungry and energetic we feel and whether our bodies burn fat or store it. The pancreas synthesizes a hormone called insulin that is responsible for transporting blood glucose into cells. When insulin is synthesized, our bodies stop burning fat and start storing it. Consumption of high GI foods cause the pancreas to produce a large amount of insulin – this insulin spike results in the transport of too much blood glucose out of the blood. As a consequence, the blood glucose level drops below normal, causing us to feel tired and hungry for something with a high sugar content, starting the cycle all over again. In contrast, consumption of low GI foods results in lower but more sustained increases in blood glucose and reduced insulin release.
More information can be found at the official website of the Glycemic Index and GI database.
- Jenkins et al. Glycemic index of foods: a physiological basis for carbohydrate exchange. Am J Clin Nutr. 1981 Mar;34(3):362-6.
- Jenkins and Jenkins. Dietary fiber and the glycemic response. Proc Soc Exp Biol Med. 1985 Dec;180(3):422-31.
- Jenkins et al. Glycemic index: overview of implications in health and disease. Am J Clin Nutr. 2002 Jul;76(1):266S-73S.